This invention relates to the field of frequency synthesis. More particularly, this invention relates to direct synthesis apparatus utilizing a minimum number of filter circuits to provide a desired number of output frequencies and a desired frequency resolution.
As is known in the art, the term "frequency synthesis" encompasses various methods and apparatus wherein a frequency conversion process is utilized to translate the signal frequency of one or more reference signals to a generally large number of output signal frequencies that are relatively stable in frequency and relatively pure in spectral content, each of which output frequencies can be individually selected as the frequency of the frequency synthesizer output signal. As is further known, frequency synthesis is utilized in a wide range of applications in which a number of stable, spectrally pure signal frequencies are required. For example, to eliminate the large number of crystal controlled oscillators that would otherwise be required, frequency synthesis is often utilized in channelized communication systems, especially those communications systems that employ frequency-hopping techniques, test and instrumentation systems, and frequency-agile radar systems.
Three basic frequency synthesis techniques have become known within the art and are commonly identified as direct frequency synthesis, indirect frequency synthesis and digital frequency synthesis. In apparatus employing direct frequency synthesis, the output frequencies are obtained directly from the frequency of one or more reference signals by the operations of mixing, filtering, frequency multiplication and frequency division. Indirect frequency synthesizers derive the output frequency from a secondary oscillator that is either phase-locked or frequency-locked to the frequency of a reference signal. In contrast, digital frequency synthesis is a sampled data technique in which a reference signal determines an invariant sampling interval and real-time digital computation is employed to calculate signal amplitudes, which when filtered, result in an analog signal of the desired frequency.
Each of the basic frequency synthesis techniques exhibits performance characteristics that often make a particular type of frequency synthesizer preferable in a particular design situation. For example, in comparison with apparatus utilizing digital frequency synthesis or direct frequency synthesis, apparatus employing indirect frequency synthesis exhibits relatively slow switching times with respect to changing between the available output frequencies. On the other hand, digital frequency synthesis generally requires a rather complex digital processor such as a programmed digital computer to compute the amplitude of the desired output signal at each of the predetermined sampling times. Accordingly, the use of direct frequency synthesis is often desirable or necessary.
One of the major problems with prior art direct synthesis has been the large number of rather complex multiple pole filters that are generally required to provide the desired number of output signal frequencies with the desired spectral purity. For example, in one basic implementation of direct frequency synthesis, the desired signal frequencies are developed by a harmonic generator, which provides a signal having signal components at frequencies harmonically related to that of a reference signal, and a harmonic selector, which includes a set of selective filters, each of which passes a desired output frequency. Since a large number of output frequencies are often required such apparatus presents a filtering requirement that necessitates an equally large number of structurally complex, multiple pole filters.
Direct frequency synthesis using iterated identical stages, sometimes called "mix-filter-divide" synthesis is another widely utilized direct synthesis technique. See, for example, chapters 2 and 3 of Frequency Synthesis: Techniques and Applications, edited by Gorski-Popiel, published by the IEEE Press, 1975, Library of Congress catalogue card #74-82502, and U.S. Pat. No. 3,838,355 issued to Papaieck. In apparatus utilizing iterated identical stages, a number of identical stages share a common set of K reference signals having frequencies that are spaced-apart from one another by a frequency increment .DELTA.f. Each stage includes a cascade arrangement of a mixer, a bandpass filter, a frequency divider, and a lowpass filter. An RF switch at each stage is operable (usually in response to a digital control signal) to apply a selected one of the reference signals to one input port of the mixer of that stage, with the second input port of the first circuit stage being supplied a signal having a frequency that is fractionally related to the frequency of one of the reference signals and the second input port of the mixer of each of the other stages being supplied with the output signal of the preceeding stage. The passband of the bandpass filter includes a desired mixer output frequency (usually the first sum frequency), which is divided by the factor K in the frequency divider. The lowpass filter eliminates harmonics in the output signal of the divider. As is known, such a circuit arrangement provides a plurality of signal frequencies K.sup.n where n is the number of stages employed with a frequency resolution of .DELTA.f/K.sup.N-1.
Although a large number of signal frequencies can be supplied by apparatus utilizing iterated indentical stages, such apparatus requires a set of K precise reference signals either generated within the apparatus or applied from external sources. Further, such apparatus requires a relatively large number of digital and RF circuit components, including a large number of multiple pole filter arrangements.
Although such prior art direct frequency synthesis techniques and apparatus provides satisfactory operation in many situations, a need exists for less complex apparatus. Accordingly, it is an object of this invention to provide apparatus for direct frequency synthesis which is of relatively simple structure, often requiring only a single mixer to supply a desired number of frequencies.
It is another object of this invention to provide apparatus for the direct synthesis of a large number of signal frequencies from a single reference signal.
It is still another object of this invention to provide apparatus for directly synthesizing a large number of signal frequencies from a single reference signal wherein minimal signal filtering apparatus is required.